Deep tissue imaging with short-wave infrared light and adaptive optics

197 pagesImaging with high spatial resolution and high specificity within intact tissues at depth has long been a critical research objective for implementation in biological studies. The development of imaging tools with the capability of deep imaging at cellular resolution would allow for more realistic and complicated biological hypotheses to be tested in their natural environment – intravitally. The most challenging aspects of such tool developments involve light scattering and aberration, which cause the light to distort along its propagation direction, limiting both its imaging depth and resolution. This thesis attempts to provide several solutions to overcomes these challenges. To overcome light scattering, imaging within the short-wave infrared region (SWIR, wavelength 1 – 2.5 micrometers) is explored in chapters 2-4. In chapter 2 and 3, reflectance confocal and fluorescence confocal microscopy are demonstrated providing 2–4 times deeper penetration than any previously reported work and preclude the possibility of using one-photon confocal microscopy for deep imaging, a method that has been rarely discussed. Furthermore, a study on the impact of staining inhomogeneity on the depth limit of fluorescence confocal microscopy also demonstrated the potential of confocal microscopy combined with SWIR and low staining inhomogeneity to achieve unprecedented imaging depth. After demonstrating the deep imaging capability of one-photon imaging at depth with a SWIR light source, a multimodal system combining three-photon, third-harmonic, and optical coherence microscopy (OCM) is demonstrated in chapter 4. This multimodal system was able to achieve simultaneous imaging depth comparable to imaging with multiple contrast mechanisms in terms of the fluorescence, the harmonic signal, and the backscattering. Furthermore, this multimodal system provided complementary information about the mouse in vivo and represented a powerful intravital biological imaging tool. To overcome light aberration, adaptive optical methods are demonstrated in chapters 5-7. In chapter 5, a sensorless, adaptive optics, and indirect wavefront sensing system is demonstrated to improve SWIR-excited three-photon imaging, achieving about 7x signal enhancement in the mouse hippocampus area. This method is based on using the nonlinear three-photon fluorescence signal as feedback and involves light exposure during the optimization process. To reduce light exposure, a more direct wavefront sensing method is explored using a SWIR OCM system to directly sense the complex field of a biological sample in chapter 6. The advantage of this system, including its potential high-speed wavefront sensing and offline wavefront estimation, and its limitations with respect to phase stability are discussed. Finally, in chapter 7, a direct wavefront sensing method based on a cheap silicon wavefront sensor is presented. This method provides a convenient approach for aberration measurement with any experiment that involves SWIR ultrafast laser. This thesis shows the great promise to achieve high-resolution deep tissue imaging at a larger depth by combining longer wavelength at short-wave infrared region and adaptive optics. It is anticipated that this thesis work will open doors to much more exciting biological research in the near future